Stain and corrosion resistant tin surface and method of producing the same



Potenieci Aug. 1?, &4

SURFACE AND ME'EHQE WE SAME Frank E. Roth, l'dellievue, E

No Drawing. Application Februeny 226, will, Serial No. 3%,963

(Gil. 141M) 9 Claims.

This invention relates to the. treatment of sun feces oi tin or tin elloys in order to make them. stain and corrosion resistant. More particularly it relates to the treatment of the surfaces of tin plate or tin plate containers to render them stain and corrosion resistant without materially affect-- ing their appearance,

Tin, in general, may he recorded as a corrosion resistant metal end, section, has found wldescreed application in the arts as a. protective costing: ior steel end other metals. Tin coated steel is universolly accepted es the most suitable materiel tor the menuieoture of containers for food. and merry other products. However, a. tirined'suriece is subject to e. numberv of defects since tin is not absolutely inert but will react to 2. limited extent with certein types of materials. These reactions ere not particularly serious in themselves but insoior us my oi them result in discoloration of either the tin surface or the food product packed in the container, they ore objectionable. Some typicol examples of rose tions oi this sort and the defects resulting there from ere:

(ll Guidotion of tin piste during storage or in the baking operation during enameling or lccouering, this resulting in a. type of discoloretion known as yellow stain. This stain is generclly recorded as an interference film of tin oxide, Electrodeposited tin plate produced commercielly is especially subject to this type of discoloration.

(2) mscolorotion of the exterior of tin cans during the retort processing of food products, this being usually a. purplish stain and may be classed as steam oxidation. It seldom occurs if the air is completely swept out oi the retort.

(3) Discoloretion oi the interior of tin cans by foods such as corn, peas, beans, corned beef, sec foods and many other products which contain sulphur hearing protein matter, reactions of this type resulting in purple or black sulphide areas on the tin plate. With some foods, such as corn, smell particles of the food itself are darkened.

i) Etching of the tin by corrosive food such as ppkin, spinach, hominy, sweet potatoes, or acid fruits, in some cases etching being accompooled by discoloration. In many foods etching is not very severe and hence is not troublesome. Where etching is severe, however, it is objectionable from an appearance standpoint and for other reasons.

- (5) Decoloration or bleaching of fruits and vegetables by reaction between the coloring matter oi the fruit and the tin, this being especially true oi red fruits.

(6) Reaction between beer and metallic tin,

this resulting in turbidity of the beer and a deteriorction of the flavor.

Detects of type 3, l, 5 end 6 and others of a slur nature have resulted in the widespread practice or enamelinc the interior oi tin cans for many uses. Although enemeiing has been fairly efiective in comboting these defects, it is ex pensive end in the case oi corrosive fruits there is elwoyc the possibility of e break, in the enamel coat with the result that corrosion will be con centrotecl on that spot and perforation of the con may occur. llurther, most enamels require barking at elevated. temperatures (l00-l25 Fahrenheit) and during this process yellow staining of the tin plate is opt to occur. Enem= cling is not efiective in preventing oxidation of the tin surioce, as evidenced by the feet that yellow stein will iorm beneath the enamel during baking.

Attempts have been mode to nrevent dork sul= phide stains by methods other then ensmelin'g. Schutte, in the United t'ltetes Patent; No. 2,62%,951, discloses the prevention of sulphide cliscoloretions by oxidizing the surfaces of tin con= telnet-s by meons oi c. permanganate solution. This method. While successiul, is subject to e. number of limitotions. Cools end Romine, in United States potent application Serial No. 2%,913, discloses the use oi electrochemical snociic oxidation es c, more satisfactory means for producing the same result. Although these processes are more or less successful inpreventins sulphide discoloration of tin by nearly neutral foods, they are inefiective in preventing etch ing by foods oi o more corrosive nature.

Fromthe ioregoins', it is apparent that no general method exists for treating tin surfaces to eliminate all the previously described defects. The practice oi enemeling is the closest approach to such a. generel method, and is practical in only as limited number oi cases and will not prevent yellow st Generally stated, the present invention consists in treating tin plate, tin or tin alloy surfaces with on. acid solution containing phosphate ions. The appearance oi the tin surface is not changed by the treetment out the tree-ted surfeces are readily distinguished by, their resistance to oxidation, resistance to ettock by dilute acids and resistance to staining by food products, especially those foods which contain sulphur compounds. Since tin and tin oxides are known. to react with phosphoric acid to produce highly insoluble and relatively inert compounds, I believe it is not unreasonable to assume that as a. result of the treatment the tin surface is provideo. with e. dense superficial protective film of an insoluble phosphate compound which is a. substantially integral pert oi said surface. Such c. protective film would function as a. barrier preventing contact of the tin and substance with which tin normally reacts and thus would prevent the reactions previously mentioned.

1. A tin surface which carries or is provided with a film of oxide can be effectively treated in a hot acidified solution of a soluble phosphate compound,whereas the treatment of a chemically clean tin surface in such a solution is relatively ineffective. I believe this difference is due to the fact that phosphoric acid reacts with stannic oxide to produce stannic oxyphosphate while the reaction of this acid with metal lic tin produces a stannous phosphate.

2. A chemically clean tin surface'can be effectively treated if an oxidizing agent is added to the treating solution. The presence of the oxidizing agent demands the formation of a stannic compound. The only readily formed quadravalent tin compound of phsphoric acid, known to me, is stannic oxyphosphate.

3. The treatment of a "chemically clean" tin surface with a solution containing an oxidizing ace tension of the treating solution.

seams? feta-ta handling in subsequent manufacturing steps.

In addition to the ingredients already mentioned, it has been found desirable to add a. 1 quantity of a wetting agent to reduce the The wetting agent promotes immediate contact of the tinned surface and the. solution, thus faciliagent is more effective than the treatment of an oxidized" tin surface. In. the latter case the conditions are such that a stannous phosphate as well as stannic phosphate could be formed, hence I conclude that conditions favorable to the formation of a film consisting substantially of stannic oxyphosphate, are to be preferred.

As indicated in the above discussion of the nature of the protective film, two methods may be practiced-under the present invention. The first and simpler method requires the presence of an oxide film on the surface to be treated. Pretreatment of the surface in the first method is not involved, however, since sufflcient oxide film is present on all tin surfaces, even immediately after manufacture, to meet this requirement. The product of the first method is a tin surface which will not "yellow stain" and is more resistant to other types of defects than are ordinary tin surfaces but the treated surface cannot be said to be substantia1lyfree from the latter.

The product of the simplified treatment is applicable only where freedom from yellow stain is the important characteristic and the conditions of service with regard to defects of other types are not severe.

In practicing the present invention in its simplest form the tin surface is immersed for a period 'of 5-10 seconds in a hot (WW-212 Fahrenheit) solution containing phosphate anions and alkali metal cations, for exple of the following composition:

Di-sodium phosphate, Nazi 121330 lilo gins/liter Ortho-phosphoric acid, i." (85%) rnlJliter Water "To make one liter tating the film formation. Any wetting agent which is stable in an acid solution may be used. An example of a wetting agent which has been used successfully is Duponol 80," a compound currently marketed under this name" by E. I. du Pont de Nemours & Company.

While in the example cited above, di-sodi phosphate is the main ingredient of the treat ing solution, I do not wish to be limited to its use. This material is chosen as preferable because it is readily soluble and is a cheap source of phosphate ion. The sodium ion does not enter into the reactions which take place at the tin surface. Other soluble phosphate compounds could be suulbstituted without affecting the ultimate res t.

Regarding acidity of the treating solution, I have observed that the effectiveness of the treatment increases as the solution is made more acid. The increase is marked over the pH range of 6.8 (minimum acidity) to 3.5. Reducing the pH below 2.5, however, has but very slight benefit and may increase the tendency for etching of the tin to occur. For the best results compatible with economic operation, I recommend a pH range of 3.5 to 2.5.

In adjusting the acidity of the solution, I have specified the use of ortho-phosphoric acid, l-hPOi. Other acids, however, may be used for this purpose. Ortho-phosphoric acid was chosen as it permits easy regulation of the pH within the range 3.5 to 2.5 and also contributes to the phosphate content of the solution.

As to operating temperature of the solution, I have observed that the eificiency of the treatment improves with increasing temperature; also, that the time required for treatment decreases as the temperature of the solution is increased. Hence, in practice, I prefer to operate the solution as hot as possible, namely, at or near the boiling point of the solution. 1

Concentration limitations of the treating solution are discussed below:

1. Di-sodium phosphate, NazHPOi 121110; the concentration of this ingredient may be varied over a range of 25 to.200 grams per liter. Less than 25 grams per liter is insufllcient for acceptable results. The effectiveness of the treatment improves as the concentration of this material is increased. .The improvement between and 200 grams per liter, however, is slight, and increasing the concentration beyond 200 grams per liter is therefore uneconomical.

2. Ortho-phosphoric acid, HaPOs: The amount required depends on the pH desired as well as the concentration of di-sodium phosphate used. If it is desired to operate within the recommended pH range (3.5 to 2.5), the amount of acid required varies directly with the di-sodium phosphate content in the apprommate ratio ofaqueous Mme-phosphate containing solution in me presence of (an oxidizing agent, and produc= ing by the simple ersion an invisible stanmo oxyphosphate on the surface.

7. A method of treating a. tin surface to ren der the same chemically inert, which comprises making the surface chemically clean and produc ing on the chemicolly clean surface a protective, invisible film consisting of substantially pure stsmnio ohosphate by simplyvimmersing the said surface in an aqueous solution containing a sodium phosphate, orthophosphoric acid and chrome oxide.

3. A container having 2, surmce of metallic tin A tin plate stock adapted to be fabricated into containers and having a protective coating on the tin plate which coating is an invisible film consisting of pure stannic oxyphosphate, produoed by simply immersing the said tin plate stock in an aqueous solution containing a sodium phosphate, orthophosphoric acid, and chromic omde. 

